In these days, it is not unusual that a family owns a plurality of cars or motorcycles. Since the family members have a plurality of cars or motorcycles, they also have a plurality of keys for operating each car. Moreover, when they go outside by car, they carry a house front door key and the like as well as a car key.
However, it is troublesome to own a plurality of keys in this manner and further to often confirm which key is to be used.
Moreover, in some case, the key is carelessly forgotten to be removed. In a delivery car or the like, the key is sometimes intentionally kept without being removed because it is troublesome to repeatedly stop/start the engine. In this case, the car may be stolen. Furthermore, if the stolen car is used in a crime, it may cause trouble for other people as well as the damage of the car theft.